Power-up reset conditioned on direction of voltage change

ABSTRACT

A system which includes a microprocessor (or microcontroller) and an auxiliary chip which monitors the system power supply voltage and performs related functions for the microprocessor. In one mode of operation (for use with a low-power CMOS processor), the auxiliary chip sends an interrupt to the microprocessor when the power supply falls to a first level, and also resets the microprocessor when the supply voltage reaches a second preset level on the way up (i.e. while power is being restored). In a selectable second mode of operation (for use with NMOS processor), the auxiliary chip resets the processor when the power supply is on the way down.

This is a continuation of application Ser. No. 283,268, filed Dec. 9, 1988, now abandoned.

PARTIAL WAIVER OF COPYRIGHT

All of the material in this patent application is subject to copyright protection under the copyright laws of the United States and of other countries. As of the first effective filling date of the present application, this material is protected as unpublished material.

However, permission to copy this material is hereby granted to the extent that the copyright owner has no objection to the facsimile reproduction by anyone of the patent document or patent disclosure, as it appears in the United States Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

CROSS-REFERENCE TO OTHER APPLICATIONS

The following applications of common assignee contain related subject matter, and are believed to have effective filling dates identical with that of the present application:

Ser. No. 283,265, filed Dec. 9, 1988, entitled "SLEEP COMMAND CONDITIONED BY TIMING WINDOW DERIVED FROM STROBE PIN", now abandoned;

Ser. No. 283,267, filed Dec. 9, 1988, entitled "MICROPROCESSOR AUXILIARY WITH ABILITY TO BE QUERIED RE POWER HISTORY" now abandoned;

all of which are hereby incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to low-power systems and subsystems employing microprocessors, and to integrated circuit elements which help to manage the low-level operation of a microprocessor.

The very rapid progress of integrated circuit complexity generally, and the general use of CMOS processing, have permitted a huge increase in the functionality which can be included in a very compact portable system. The availability of low-power LCD displays has further speeded the advance of such systems. However, power supply capabilities have not advanced as rapidly. Battery technology has provided a relatively slow increase in the amount of energy which can be stored per unit weight (or per unit volume). Thus, in order to provide complex functionality in a small portable module, a very high degree of power efficiency has become an enabling technology.

A separate line of technological progress is the increasing use of batteries, in integrated circuit packages or in very small modules, to provide nonvolatile data retention. Here the driving concern is not the system power budget, but reliability and robustness. The availability of battery backup can be used to ensure that power outages or powerline noise cannot cause loss of data (including configuration data). For example, modern semiconductor technology has provided solid-state memories with such low standby power requirements that a single coin-sized battery can power the memory for ten years of lifetime or more. Such memories are already commercially available.

Low-power microcontrollers have also been commercially available in recent years. An unusual example of such a microcontroller is the DS5000 Soft MicroController™. (This integrated circuit and its data sheet are available from Dallas Semiconductor Corporation, 4350 Beltwood Parkway, Dallas, TX. 75244, and are both hereby incorporated by reference.) The DS5000 is a microcontroller which has a small battery packaged with it, to provide nonvolatility. Microprocessors and microcontrollers of this kind are extremely useful, since the internal memory of the microprocessor is always preserved. Therefore, the microprocessor can be programmed to "learn" while in service, or to internally store a parameter set which is adjustable throughout the lifetime of the microprocessor. However, aside from their nonvolatility, such microprocessors are typically not the highest-performing microprocessors. Thus, a user who needs nonvolatility may need to make some difficult choices.

The present invention provides an auxiliary integrated circuit, which can interface with a microprocessor (or other complex random logic chip) in a way which improves the microprocessor's power management during power-up and power-down transitions.

In the presently preferred embodiment, this auxiliary chip provides all necessary functions for power supply monitoring, reset control, and memory back-up in microprocessor based systems. A precise internal voltage reference and comparator circuit monitor power supply status. When an out-of-tolerance condition occurs, the microprocessor reset and power fail outputs are forced active, and static RAM control unconditionally write protects external memory. The auxiliary chip also provides early warning detection by driving a non-maskable interrupt at a user defined voltage threshold. External reset control is provided by a pushbutton reset input which is debounced and activates reset outputs. An internal timer also forces the outputs to the active state if the strobe input is not driven low prior to time out. Reset control and wake-up/sleep control inputs also provide necessary signals for orderly shut down and start up in battery backup and battery operate applications.

The auxiliary chip provided by the present invention can be used with a very wide variety of different microcontrollers and microprocessors. Two significantly different types must be distinguished:

For low-power battery-backed CMOS microcontrollers and microprocessors (such as the DS5000), the microprocessor should not be reset when power is failing (because such a reset will wake up the microprocessor, and cause it to draw power).

For NMOS microprocessors, and for CMOS microprocessors or microcontrollers which do not have access to a backup power supply, it is desirable to reset the processor as soon as possible when the power supply is failing, and keep it in reset until the power supply begins to recover. (Bringing the microprocessor under control early helps minimize power consumption, and helps to avoid random outputs from the microprocessor.)

A battery-backed microprocessor should preferably go into its "stop" mode when power goes down. However, the microprocessor alone does not normally know when it has been switched over to battery backup.

Another of the innovative teachings set forth in the present application is an auxiliary chip which sends a reset command to an associated microprocessor at a certain voltage level, but only if the direction of voltage change indicates that the power supply is coming up, and not going down. This permits the microprocessor to be reset, and resume operation, as early as possible when power is restored, but prevents a nonvolatized microprocessor from being reset during its low-power state.

This auxiliary chip, and systems or subsystems which use this auxiliary chip, provide at least the following advantages:

Holds microprocessor in check during power transients;

Halts and restarts an out-of-control microprocessor;

Monitors pushbutton for external override;

Warns microprocessor of an impending power failure;

Converts CMOS SRAM into nonvolatile memory;

Unconditionally write protects memory when power supply is out of tolerance;

Consumes less than 100 nA of battery current;

Can control an external power switch for high current applications;

Provides orderly shutdown in nonvolatile microprocessor applications;

Supplies necessary control for low power "stop mode" in battery operate hand held applications.

A further advantage of this auxiliary chip is that it provides designers with a greatly increased range of options. This auxiliary chip permits system designers to obtain many of the advantages of a specialized low-power microprocessor (such as the DS5000), while using a different microprocessor which has higher-speed, or more versatility, or compatibility with some existing software base, or special adaptation for some special purpose.

Thus, systems which include the combination of an auxiliary chip as described with a general-purpose microprocessor can have advantages including robustness in the face of power-supply crashes or glitches, and program resumption which appears (to the user) to be continuous with the program's operation at the moment when the machine was turned off (depending on how state-save operations are interwoven with software execution).

Normally, when it is desired to put a microprocessor into a known state, this is done by activating a reset. Some microprocessor architectures have reset lines running to every gate on the chip, so that a reset command will instantly reset every logical element to the known state. However, some architectures do not. For example, in the Intel 8051 architecture, several cycles are necessary after the reset command, to clock all of the logical elements on the chip into the known state. (This architecture is used not only in Intel's 80C51 microprocessor, but also in any other microprocessor which is to be compatible with this widely-used architecture.) For example, a simple example of a logic block which would require multiple cycles to reset would be a shift register, with a reset only at the input of the shift register. In this (hypothetical) case, it can be seen that, even after the reset command has provided a known state in the first stage of the shift register, unknown data may still exist in the following stages. Therefore, a series of clock commands must be provided, to propagate the known state all the way through the shift register.

Alternatively, if it is necessary to save the state of a microprocessor entering power-down, this can be done separately. For example, a "shadow" memory or register can be used to track the status of various on-chip registers, etc. Similarly, if desired, portions of on-chip memory can even be used as "shadow" scratch pad, to preserve some state information during such power-down operations.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

FIG. 1 shows a typical example of the power monitor, watchdog timer, and pushbutton reset.

FIG. 2 shows how the high impedance input at the IN pin allows for a user to define a sense point, using a simple resistor voltage divider network to interface with high voltage signals.

FIG. 3 shows a typical nonvolatile SRAM application.

FIG. 4 depicts the three negative pulses on the IN pin which are used to invoke the freshness seal.

FIG. 5 shows how the external supply voltage is switched by discrete transistors, controlled by power-fail signal PF and its complement PF*.

FIG. 6 shows the power-down timing relations which result, in the presently preferred embodiment, when the reset control input (RC) has been tied to V_(CCO).

FIG. 7 shows the power-down timing relations which result, in the presently preferred embodiment, when the reset control input (RC) has been tied to ground.

FIG. 8 shows the power-up timing relations which result, in the presently preferred embodiment, when the reset control input (RC) has been tied to ground.

FIG. 9 shows the power-up timing relations which result, in the presently preferred embodiment, when the reset control input (RC) has been tied to V_(CCO).

FIG. 10 shows the signal timing relations which permit sleep mode to be entered, and FIG. 11 shows the signal timing relations which permit the chip to awaken from sleep mode.

FIG. 12 shows the timing relation between the NMI* and ST* signals.

FIG. 13 shows the overall organization of the auxiliary chip of the presently preferred embodiment.

FIG. 14 shows the critical points on the curve of power supply voltage, when the power supply voltage is falling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment. However, it should be understood that this embodiment is only one example of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others.

In the following description, the following pin and signal names may be referred to:

    ______________________________________                                         V.sub.BAT     +3 Volt Battery Input                                            V.sub.CCO     Switched SRAM Supply Output                                      V.sub.CC      +5 Volt Power Supply Input                                       GND           Ground                                                           PF            Power Fail (Active High)                                         PF*           Power Fail (Active Low)                                          WC/SC*        Wake-Up Control (Sleep)                                          RC            Reset Control                                                    IN            Early Warning Input                                              NMI*          Non Maskable Interrupt                                           ST*           Strobe Input                                                     CEO*          Chip Enable Output                                               CEI*          Chip Enable Input                                                PBRST*        Push Button Reset Input                                          RST*          Reset Output (Active Low)                                        RST           Reset Output (Active High)                                       ______________________________________                                    

POWER MONITOR: The auxiliary chip employs a bandgap voltage reference and a precision comparator to monitor the 5 volt supply (V_(CC)) in microprocessor based systems. When an out-of-tolerance condition occurs, the RST and RST* outputs are driven to the active state. The V_(CC) trip point (V_(CCTP)) is set, for 10% operation, so that the RST and RST* outputs will become active as V_(CC) falls below 4.5 volts (4.37 typical). The V_(CCTP) for the 5% operation option is set for 4.75 volts (4.62 typical). The RST and RST* signals are excellent for microprocessor control, as processing is stopped at the last possible moment of within-tolerance V_(CC). On power up, the RST and RST* signals are held active for a minimum of 40 ms (60 ms typical) after V_(CCTP) is reached to allow the power supply and microprocessor to stabilize. The mode of operation just described (and shown in FIGS. 7 and 8) is achieved if the reset control pin (RC) is connected to GND. Alternatively, by connecting the reset control pin RC to voltage V_(CCO), a different mode of operation can be achieved. This different mode is shown in FIGS. 6 and 9, and is described below.

WATCHDOG TIMER: The auxiliary chip also provides a watchdog timer function by forcing the RST and RST* signals to the active state when the strobe input (ST*) is not stimulated for a predetermined time period. This time period is set for 220 ms typically with a maximum time-out of 300 ms. The watchdog timer begins timing out from the set time period as soon as RST and RST* are inactive. If a high-to-low transition occurs at the ST* input prior to time-out, the watchdog timer is reset and begins to time out again. To guarantee that the watchdog timer does not time-out, a high-to-low transition must occur at or less than 150 ms from watchdog timer reset. If the watchdog timer is allowed to time out, the RST and RST* outputs are driven to the active state for 40 ms minimum. The ST* input can be derived from microprocessor address, data, and/or control signals. Under normal operating conditions, these signals would routinely reset the watchdog timer prior to time out. If the watchdog timer is not required, it may be disabled by permanently grounding the IN input pin which also disables the NMI* output (described below). If the NMI* signal is required, the watchdog may also be disabled by leaving the ST* input open. The watchdog timer is also disabled as soon as the IN input falls to V_(TP) or, if IN is not used and grounded, as soon as V_(CC) falls to V_(CCTP). The watchdog will then become active as V_(CC) rises above V_(CCTP) and the IN pin rises above V_(TP).

PUSH-BUTTON RESET: An input pin is provided on the auxiliary chip for direct connection to a push-button. The push-button reset input requires an active low signal. Internally, this input is debounced and timed such that the RST and RST* outputs are driven to the active state for 40 ms minimum. This 40 ms delay begins as the pushbutton is released from low level. A typical example of the power monitor, watchdog timer, and pushbutton reset is shown in FIG. 2.

NONMASKABLE INTERRUPT: The auxiliary chip generates a non-maskable interrupt NMI* for early warning of power failure to a microprocessor.

A precision comparator 110 monitors the voltage level at the input pin IN relative to a reference generated by the internal bandgap. The IN pin is a high impedance input allowing for a user defined sense point using a simple resistor voltage divider network (FIG. 3) to interface with high voltage signals. This sense point may be derived from the regulated 5 volt supply, or from a higher DC voltage level closer to the AC power input. Since the IN trip point V_(TP) is 2.54 volts, the proper values for R₁ and R₂ can easily be determined. Proper operation of the auxiliary chip requires that the voltage at the IN pin be limited to 5 volts maximum. Therefore, the maximum allowable voltage at the supply being monitored (V_(MAX)) can also be derived as shown. A simple approach to solving this equation is to select a value for R₂ of high enough impedance to keep power consumption low, and solve for R₁. The flexibility of the IN input pin allows for detection of power loss at the earliest point in a power supply system, maximizing the amount of time for microprocessor shut-down between NMI* and RST or RST*. When the supply being monitored decays to the voltage sense point, the auxiliary chip drives the NMI* output to the active state for a minimum of 200 microseconds, but does not hold it active. If the pin is connected to V_(CC), the NMI* output will pulse low as V_(CC) decays to V_(CCTP) if RC pin at ground (see reset control section). NMI* will not pulse low if the RC pin is connected to V_(CCO). The NMI* power fail detection circuitry also has built in time domain hysteresis. That is, the monitored supply is sampled periodically at a rate determined by an internal ring oscillator running at approximately 47 kHz (20 ms/cycle). Three consecutive samplings of out-of tolerance supply (below V_(SENSE)) must occur at the IN pin to active NMI. Therefore, the supply must be below the voltage sense point for approximately 60 ms or the comparator will reset.

The NMI* signal has been defined, in the presently preferred embodiment, as a pulse, rather than a level, because a constant output would keep some microprocessors from going into their lowest-power mode. Thus, the microprocessor cannot simply scan the NMI* signal to see where the power supply voltage level is.

However, the microprocessor can query the auxiliary chip to see where the power supply level is. Whenever the auxiliary chip receives a pulse from the microprocessor on the ST* line, it will return a pulse to the microprocessor on the NMI* line, but only if the system supply voltage is less than the required to trip the NMI* interrupt.

FIG. 14 is a simplified version of FIG. 7, which shows this relationship more clearly. In this diagram, V₁ refers to the voltage at which the auxiliary chip generates an interrupt (on line NMI*, in the presently preferred embodiment); voltage V₂ is the voltage at which the auxiliary chip generates a reset (this is equal to voltage V_(CCTP), in the presently preferred embodiment); and voltage V₃ is the voltage at which comparator 130 connects the internal VCC to V_(BAT) rather than to V_(CCI) (which is the externally supplied power voltage, as opposed to the on-chip supply V_(CC)). Correspondingly, several voltage domains are indicated:

    in domain 1, V.sub.CCI >V.sub.1 ;

    in domain 2, V.sub.1 >V.sub.CCI >V.sub.2 ;

    in domain 3, V.sub.2 >V.sub.CCI >V.sub.3.

The microprocessor can send a query to the auxiliary chip by pulsing the strobe pin ST*. When this occurs, the auxiliary chip will reply with a pulse on line NMI* if the supply level is then in zone 2, but not If the power supply level is in zone 1. Thus, the microprocessor can use this exchange to recognize whether it is in zone 2. This is important because the watchdog operation is turned off in zone 2, so that otherwise it might be possible for a stuck condition to occur.

MEMORY BACKUP: The auxiliary chip provides all necessary functions required to battery back up a static RAM. First, a switch is provided to direct power from the incoming 5 volt supply (V_(CC)) or from a battery (V_(BAT)) whichever is greater. This switched supply (V_(CCO)) can also be used to battery back CMOS microprocessors. (The reset control and wake control sections provide further discussion regarding nonvolatile microprocessor applications.) Second, the same power fail detection described in the power monitor section is used to inhibit the chip enable input (CEI*) and hold the chip enable output (CEO*) to within 0.3 volts of V_(CC) or battery supply. This write protection mechanism occurs as V_(CC) falls below V_(CCTP) as specified previously. If CEI* is low at the time power fail detection occurs, CEO* is held in its present state until CEI* is returned high, or if CEI* is held low, CEO* is held active for t_(CE) maximum. This delay of write protection until the current memory cycle is completed prevents the corruption of data. If CEO* is in an inactive state at the time of V_(CC) fail detection, CEO* will be unconditionally disabled within t_(CF). During nominal supply conditions CEO* will follow CEI* with a maximum propagation delay of 20 ns.

FIG. 4 shows a typical nonvolatile SRAM application. If nonvolatile operation is not required, the battery input pin V_(BAT) must be grounded. In order to conserve battery capacity during storage and/or shipment of a system, the auxiliary chip provides a freshness seal to electronically disconnect the battery.

FIG. 5 depicts the three pulses below ground on the IN pin required to invoke the freshness seal. The freshness seal will be disconnected and normal operation will begin when V_(CC) is next applied to a level above V_(BAT).

POWER SWITCHING: For certain high current battery backup applications, the 5 volt supply and battery supply switches internal to the auxiliary chip may not be large enough to support the given load within significant voltage drop. For these applications, the PF and PF* outputs are provided to gate external switching devices to switch supply from V_(CC) to battery on power down and from battery to V_(CC) on power up. The transition threshold for PF and PF* is set to the external battery voltage V_(BAT) (see FIG. 6). The load applied to the PF pin from the external switch will be supplied by the battery. Therefore, this load should be taken into consideration when sizing the battery.

RESET CONTROL: Two modes of operation on power down and power up are available, depending upon the level of the reset control (RC) input pin. The level of this pin distinguishes timing and level control on RST, RST*, and NMI* outputs for volatile processor operation versus non-volatile battery backed (or battery operated) processor applications.

With the RC pin tied to ground, operation is as described above. This mode is used where non-volatile processor functionality is not required. The timing relations of this mode are shown in FIG. 7 (when the power goes down) and FIG. 8 (when the power is restored). Notice that upon V_(CC) going out of tolerance (at V_(CCTP)) the RST and RST* outputs are driven active (within a delay t_(RPD)) and that RST and NMI* follow V_(CC) as the supply decays. Also, on power up, RST follows V_(CC) and RST* is held active; both remain active for a time t_(RST) after V_(CC) becomes valid. NMI* will pulse low for 500 microsec maximum, and then will follow V_(CC).

With the RC pin tied to V_(CCO), operation is as shown in the timing diagrams of FIG. 6 (when the power goes down) and FIG. 8 (when the power is restored). This mode of operation is especially useful for applications in which the processor is made nonvolatile with an external source and allows the processor to power down into a "stop" mode as signaled from NMI* at an earlier voltage level.

As power goes down, RST and RST* are not forced active as V_(CC) collapses to V_(CCTP), and RST* is held at a high level by the battery as V_(CC) falls below battery potential. The NMI* output pin will pulse low for t_(NMI) following a low voltage detect at the pin of V_(TP). However, NMI* will also be held at a high level following t_(NMI) by the battery as V_(CC) decays below V_(BAT).

On power up, RST and RST* are held inactive until V_(CC) reaches power valid V_(CCTP), then RST and RST* are driven active for t_(RST). NMI* will pulse low for 500 microseconds maximum then will follow V_(CC) during the power up sequence.

Thus, once NMI* is driven active, the processor may power down into a "stop" mode and subsequently be restarted by any of several different signals. If V_(CC) does not fall below V_(CCTP), the processor will be restarted by the reset derived from the watchdog timer as the IN input rises above V_(tp). If V_(CC) falls below V_(CCTP) but not below V_(BAT), the processor will be restarted as V_(CC) rises above V_(CCTP). If V_(CC) falls below V_(BAT), the reset outputs will be forced active the next time V_(CC) rises above V_(CCTP) as shown in the power up timing diagram. If the IN pin falls below VTP during an active reset, the reset outputs will be forced inactive by the NMI* output. An additional NMI* pulse for "stop" mode control will follow the initial NMI*, by stimulation of the ST* input, at t_(STN). The pushbutton input PBRST* may be used, whenever V_(CC) is above V_(BAT), to drive the reset outputs and thus restart the processor.

WAKE CONTROL/SLEEP CONTROL: The Wake/Sleep Control input WC/SC* allows the processor to disable all comparators on the auxiliary chip, processor, and nonvolatile static RAM to maintain nonvolatility in the lowest power mode possible. The processor may invoke the sleep mode in battery operate applications to conserve capacity when an absence of activity is detected. The auxiliary chip may subsequently be restarted by a high to low transition on the PBRST* input via human interface by a keyboard, touch pad, etc. The processor will then be restarted as the watchdog timer times out and drives RST and RST* active. The auxiliary chip can also be woken up by forcing the WC/SC* pin high from an external source. Also, if the auxiliary chip is placed in sleep mode by the processor, and V_(CC) later falls below V_(BAT), the auxiliary chip will wake up the next time V_(CC) rises above V_(CCTP). That is, the auxiliary chip leaves the sleep mode as the power is falling below V_(BAT). (As noted, when the processor invokes the sleep mode during normal power valid operation, all operation on the auxiliary chip is disabled, thus leaving the NMI*, RST and RST* outputs disabled as well as the ST* and IN inputs.) The PBRST* input will also become inactive when the main battery supply falls below the IN input at V_(TP) or the backup 3 volt supply at V_(BAT). Subsequent power up with a new main battery supply will activate the RST and RST* outputs as the main supply rises above V_(CCTP). Please review the timing diagram for wake/sleep control. A high to low transition on the WC/SC* pin must follow a high to low transition on the ST* pin by t_(WC) to invoke a "sleep" mode for the auxiliary chip.

FIG. 13 shows the overall electrical organization of the auxiliary chip of the presently preferred embodiment. A first comparator 110 compares the input voltage at the IN pin with the reference voltage provided by bandgap voltage reference generator 200. The output of this comparator is connected through time delay stage 112 to one-shot 114. Thus one-shot 114 will provide a pulse on the NMI* output pin when comparator 110 sees that the voltage at pin IN has fallen below limits. (As noted, a resistive divider network would commonly be used to scale the supply voltage appropriately for this comparison.)

A second comparator 120 compares a fraction of the supply voltage input V_(CCI) (scaled by resistors 121) with the reference voltage provided by bandgap voltage reference generator 200. The output of this comparator 120 is connected, through time delay stage 122 and OR gate 410, to reset control logic 400.

Note that the output of comparator 120 is also connected (through the time delay block 122) to control a chip-enable-control gate 139, so that incoming chip-enable signals CEI* will not be passed through to signal CEO* when V_(CCI) has fallen below V_(CCTP).

A third comparator 130 compares the external VCC supply voltage input (V_(CCI)) against the battery voltage V_(BAT), and switches large transistors 132, 134, and 136 (via NAND gate 135) appropriately, to connect the external power supply output VCCO and the internal power supply lines VCC to V_(BAT) if V_(CCI) falls significantly below V_(BAT).

The NAND gate 135 also receives an input from freshness seal logic 131, so that, if the input from freshness seal logic 131 is low, transistor 136 will never turn on. In this case, if the external power supply V_(CCI) fails, comparator 130 will drive its output PF positive, turning off transistors 132 and 134, and pin V_(CCO) will be floated. This avoids any loss of battery lifetime due to drain from external devices. The freshness seal logic 131 decodes signals received on the SLP* pin, as described above, to enter or leave the freshness-seal mode.

The output of the bandgap voltage reference 200 is also used by a current source (not separately shown), which provides a temperature-independent current to the ring oscillator. This current source also provides a temperature-independent current to the voltage reference 200. The voltage reference 200 uses this current to define charging relationships, and also makes use of the output of the ring-oscillator (to chipper-stabilize the comparators). The ring oscillator 310 provides a constant-frequency output to watchdog timer 300. The watchdog timer 300 provides timing and alarm functions, such as those performed by commercially available part DS1286. (This integrated circuit and its data sheet are available from Dallas Semiconductor Corporation, 4350 Beltwood Parkway, Dallas TX 75244, and are both hereby incorporated by reference.) In particular, the watchdog timer will provide an input to OR gate 410, to generate a reset, if it counts down through its time-out limit without having received a pulse on pin ST*.

The sleep-control logic 500 receives inputs from the SLP* pin and also from the ST* pin. The outputs of this logic (not shown) can disable not only watchdog timer 300, but also are connected to diable bandgap voltage reference 200, oscillator 310, and comparators 110 and 120. Comparator 130 is not disabled, but is switched into a low-power mode. In comparator 130's low-power mode, its bias current is reduced, so that it can still detect when V_(CCI) falls below V_(BAT), it reacts more slowly.

The third input to the OR gate 410 is from the pushbutton input PBRST*, which is cleaned up by debounce logic 420. Thus, the user can manually initiate a reset of the microprocessor at any time, without power-cycling the whole system, simply by hitting a pushbutton connected to this logic input.

Thus, the reset control logic 400 can be conditionally commanded to initiate a reset by any of the three inputs just described. However, the reset control logic 400 receives external control input RC, and also is connected to see the outputs of comparators 110 and 120, to implement the logical relations described above.

The following tables give specific values for some of the voltage and timing parameters just referred to, as used in the specific context of the presently preferred embodiment. It must be understood that these specific values are given merely to provide a wealth of detail regarding the preferred embodiment, and for better understanding of FIGS. 6-9, and do not by any means delimit necessary features of the invention.

    __________________________________________________________________________     ABSOLUTE MAXIMUM RATINGS                                                       __________________________________________________________________________     VOLTAGE ON ANY PIN RELATIVE TO GROUND                                                                          1.0V to +7.0V                                  OPERATING TEMPERATURE           0 to 70 C.                                     STORAGE TEMPERATURE             -55 to +125 C.                                 SOLDERING TEMPERATURE           260 for 10 seconds                             __________________________________________________________________________     PARAMETER     SYMBOL                                                                               MIN. TYP.                                                                               MAX.                                                                               UNITS                                                                               NOTES                                    __________________________________________________________________________     A. C. ELECTRICAL CHARACTERISTICS (0 C. to 70 C., V.sub.CC = 4.5V to            5.5V)                                                                          V.sub.CC Fail Detect to RST,                                                                 t.sub.RPD  50  100 us                                            RST*                                                                           V.sub.TP to NMI*                                                                             t.sub.IPD                                                                            30   50  100 us                                            RESET Active Time                                                                            t.sub.RST                                                                            40   60  80  ms                                            NMI* Pulse Width                                                                             t.sub.NMI                                                                            200  300 500 us                                            ST* Pulse Width                                                                              t.sub.ST                                                                             20           ns                                            PB RST* @ V.sub.IL                                                                           t.sub.PB                                                                             30           ms                                            V.sub.CC Slew Rate 4.75V to                                                                  t.sub.F                                                                              300          us                                            4.25V                                                                          Chip Enable Propagation                                                                      t.sub.PD       20  ns                                            Delay                                                                          Chip Enable High to                                                                          t.sub.CF       20  ns                                            V.sub.CC Fail                                                                  V.sub.CC Valid to (RST &                                                                     t.sub.FPU      100 ns                                            RST* RC = 1)                                                                   V.sub.CC Valid to RST & RST*                                                                 t.sub.RPU                                                                            40   60  80  ms   5                                        V.sub.CC Slew 4.25V to V.sub.BAT                                                             t.sub.FB1                                                                            10           us   7                                        V.sub.CC Slew 4.25 to V.sub.BAT                                                              t.sub.FB2                                                                            100          us   8                                        Chip Enable Output                                                                           t.sub.REC                                                                            80           ms   9                                        Recovery                                                                       V.sub.CC Slew 4.25V to 4.75V                                                                 t.sub.R                                                                              0            us                                            Chip Enable Pulse Width                                                                      t.sub.CE       5   us   10                                       Watch Dog Time Delay                                                                         t.sub.TD                                                                             150  220 300 ms                                            ST* to WC/SC* t.sub.WC                                                                             0.1      50  us                                            V.sub.BAT Detect to PF, PF*                                                                  t.sub.PPF      2   us   7                                        ST* to NMI*   t.sub.STN      30  ns   11                                       NMI* to RST & RST*                                                                           t.sub.NRT      30  ns                                            RECOMMENDED D.C. OPERATING CONDITIONS (0 C. to 70 C.)                          Supply Voltage                                                                               V.sub.CC                                                                             4.5  5.0 5.5 V    1                                        Supply Voltage                                                                               V.sub.CC                                                                             4.75 5.0 5.5 V    1                                        (5% option)                                                                    Input High Level                                                                             V.sub.IH                                                                             2.0      V.sub.cc + 0.3                                                                     V    1                                        Input Low Level                                                                              V.sub.IL                                                                             -0.3     +0.8                                                                               V    1                                        IN Input Pin  V.sub.IN       V.sub.CC                                                                           V    1                                        Battery Input V.sub.BAT                                                                            2.7      4.0 V    1                                        D.C. ELECTRICAL CHARACTERISTICS (0 C. to 70, V.sub.CC = 4.5V to 5.5V)          Supply Current                                                                               I.sub.CC       5   mA   2                                        Supply Current Output                                                                        I.sub.CC01     100 mA   3                                        Supply Voltage Output                                                                        V.sub.CC0                                                                            V.sub.CC -0.3                                                                               V    1                                        Input Leakage I.sub.LI                                                                             -1.0     +1.0                                                                               uA                                            Out Leakage   I LO  -1.0     +1.0                                                                               uA                                            Output Current @ 0.4V                                                                        I.sub.OL       4.0 mA   12                                       Output Current @ 2.4V                                                                        I.sub.OH                                                                             -1.0         mA   13                                       Power Supply Trip Point                                                                      V.sub.CCTP                                                                           4.25 4.37                                                                               4.50                                                                               V    1                                        Power Supply Trip Point                                                                      V.sub.CCTP                                                                           4.50 4.62                                                                               4.75                                                                               V    1                                        (5% option)                                                                    IN Input Pin Current                                                                         I.sub.CCIN     0.1 uA                                            IN Input Trip Point                                                                          V.sub.TP                                                                             2.5  2.54                                                                               2.6 V    1                                        Battery Backup Current                                                                       I.sub.CC02     1.0 mA   4                                        Battery Backup Current                                                                       V.sub.CCO                                                                            V.sub.BAT -0.7                                                                              V    1,6                                      Battery Current                                                                              I.sub.BAT      0.1 ua   2                                        CE* and PF Output                                                                            V.sub.OHL                                                                            V.sub.BAT -0.7                                                                              V    1,6                                      Voltage                                                                        CAPACITANCE (t.sub.A = 25)                                                     Input Capacitance                                                                            C.sub.IN       5   pF                                            Output Capacitance                                                                           C.sub.OUT      7   pF                                            __________________________________________________________________________      NOTES:                                                                         1. All voltages referenced to ground                                           2. Measured with V.sub.CCO pin, CEO* pin, PF pin, and NMI* pin open.           3. I.sub.CC01 is the maximum average load which the DS1236 can supply at       V.sub.CC3V through the V.sub.CCO pin during normal 5 volt operation.           4. I.sub.CCO2 is the maximum average load which the DS1236 can supply          through the V.sub.CCO pin during data retention battery supply operation.      5. With t.sub.R = 5 us                                                         6. V.sub.CCO is approximately V.sub.BAT0.5V at 1 ua load.                      7. Sleep mode is not invoked.                                                  8. Sleep mode is invoked.                                                      9. t.sub.REC is the minimum time required before memory access to allow        for deactivation of RST and RST*.                                              10. t.sub.CE maximum must be met top insure data integrity on power loss.      11. In input is less than V.sub.TP but V.sub.CC greater than V.sub.CCTP.       12. All outputs except RST* which is 50 ua min.                          

Further Modifications and Variations

It will be recognized by those skilled in the art that the innovative concepts disclosed in the present application can be applied in a wide variety of contexts. Moreover, the preferred implementation can be modified in a tremendous variety of ways. Accordingly, it should be understood that the modifications and variations suggested below and above are merely illustrative. These examples may help to show some of the scope of the inventive concepts, but these examples do not nearly exhaust the full scope of variations in the disclosed novel concepts.

For example, the microprocessor's programming can use the power-down warning interrupt to trigger a state-save operation.

For another example, the disclosed auxiliary chip can be used with a wide variety of microprocessors, microcontrollers, or microcomputers, including ones which do and ones which do not have their own battery back-up supplies; 8-bit, 16-bit, 32-bit, or other architectures; general-purpose processors, DSPs (digital signal processors), or ASICs (application-specific integrated circuits); numeric or symbolic processors; and others.

For another example: a wide range of system contexts are enabled by the disclosed inventions, including (for example) portable computers, device controllers, desk-top computers, sub-processors which perform management functions in minicomputer, mainframe, or even supercomputer systems.

As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly their scope is not limited except by the allowed claims. 

What is claimed is:
 1. An integrated circuit, for providing power management functions to an associated integrated circuit, comprising:a voltage comparison circuit, connected to monitor an input voltage with reference to a reference-voltage source, and to output an interrupt to the associated circuit if said comparison circuit indicates that said input voltage has declined from an input voltage target to a first voltage; reset control logic, connected to operate selectably in either a first mode or a second mode, wherein,in said first mode, said reset control logic is permitted to output a reset signal to the associated circuitIF said comparison circuit indicates that an external power supply voltage has strengthened to a second voltage after previously having declined from a target voltage to a third voltage, but NOT IF said external power supply voltage has declined from said target voltage to a second voltage without having passed through said third voltage; and in said second mode, said reset control logic is permitted to output said reset signal to the associated circuit WHENEVER said comparison circuit indicates that said power supply voltage is less extreme than said second voltage; wherein said target voltage is more extreme than said first voltage, said target voltage is more extreme than said second voltage, and said second voltage is more extreme than said third voltage.
 2. The integrated circuit of claim 1, wherein said target voltage is more positive than said first voltage, said first voltage is more positive than said second voltage, and said second voltage is more positive than said third voltage.
 3. The integrated circuit of claim 1, wherein said voltage comparison circuit comprises multiple comparators.
 4. The integrated circuit of claim 1, further comprising chip-enable control logic, connected to receive an externally generated chip-enable input signal, and to provide corresponding chip-enable outputs externally only if said comparison circuit indicates that the external power supply voltage is sufficiently near said target voltage.
 5. The integrated circuit of claim 1, wherein said reference-voltage source is a bandgap-voltage-reference, and is integrated with said voltage comparison circuit.
 6. the integrated circuit of claim 1, wherein said comparison circuit monitors said input voltage and said external power supply voltage from separate connections, and said input voltage does not necessarily correspond to said external power supply voltage.
 7. The integrated circuit of claim 1, wherein said comparison circuit receives said input voltage at an input of said comparison circuit which is connected so that said input voltage necessarily external power supply voltage.
 8. The integrated circuit of claim 1, wherein said comparison circuit comprises a comparator.
 9. The integrated circuit of claim 1, wherein, in said second mode, said reset signal declines as said supply voltage declines.
 10. The integrated circuit of claim 1, wherein, in said second mode, said reset signal is powered only by said external supply voltage.
 11. The integrated circuit of claim 1, wherein said reset signal includes both true and complemented signals.
 12. An itegrated circuit, for providing power management functions to an associated integrated circuit, comprising:a power supply input connection, for connection to an external power supply; a battery input connection, for connection to a battery; a voltage comparison circuit, connected to monitor an input voltage with reference to a reference-voltage source, and to output an interrupt to the associated circuit if said comparison circuit indicates that said input voltage has declined from an input voltage target to a first voltage; reset control logic, connected to operate selectably in either a first mode or a second mode, wherein,in said first mode, said reset control logic is permitted to output a reset signal to the associated circuitIF said comparison circuit indicates that an external power supply voltage has strengthened to a second voltage after previously having declined from a target voltage to a third voltage, but NOT IF said external power supply voltage has declined from said target voltage to a second voltage without having passed through said third voltage; and in said second mode, said reset control logic is permitted to output said reset signal to the associated circuit WHENEVER said comparison circuit indicates that said power supply voltage is less extreme than said second voltage; wherein said target voltage is more extreme than said first voltage, said target voltage is more extreme than said second voltage, and said second voltage is more extreme than said third voltage; and wherein, if said external power supply voltage fails while said reset control is in said first mode, said reset control logic will hold said reset signal in an inactive state; and wherein, if said external power supply voltage is restored while said reset control is in said first mode, said reset control logic will hold said reset signal in an inactive state until said power supply voltage exceeds said second voltage, and thereafter will drive said reset signal active for at least a predetermined duration; and wherein, if said external power supply voltage fails while said reset control is in said second mode, said reset control logic will drive said reset signal active for at least some period after said power supply voltage has fallen below said second voltage; and wherein, if said external power supply voltage is restored while said reset control is in said second mode, said reset control logic will drive said reset signal active for at least some period after said power supply voltage has risen above said second voltage.
 13. The integrated circuit of claim 12, wherein said comparison circuit monitors said input voltage and said external power supply voltage from separate connections, and said input voltage does not necessarily correspond to said external power supply voltage.
 14. The integrated circuit of claim 12, wherein said comparison circuit receives said input voltage at an input of said comparison circuit which is connected so that said input voltage necessarily corresponds to said external power supply voltage.
 15. The integrated circuit of claim 12, wherein said comparison circuit comprises a comparator.
 16. The integrated circuit of claim 12, wherein, in said second mode, said reset signal declines as said supply voltage declines.
 17. The integrated circuit of claim 12, wherein, in said second mode, said reset signal is powered only by said external supply voltage.
 18. The integrated circuit of claim 12, wherein said reset signal includes both true and complemented signals.
 19. An integrated circuit, for providing power management functions to an associated integrated circuit, comprising:a power supply input connection, for connection to an external power supply; a battery input connection, for connection to a battery; a voltage comparison circuit, connected to monitor an input voltage with reference to a reference-voltage source, and to output an interrupt to the associated circuit if said comparison circuit indicates that said input voltage has declined from an input voltage target to a first voltage; a power supply output connection, connected to be driven with currentfrom said power supply input connection, if the voltage of said power supply input connection is substantially higher than the voltage of said battery connection, and from said battery connection, if the voltage of said battery connection is substantially higher than the voltage of said power supply input connection, except that said power supply output connection is not driven when said comparison circuit indicates that said power supply voltage is less than said second voltage; reset control logic, connected to operate selectably in either a first mode or a second mode, wherein,in said first mode, said reset control logic is permitted to output a reset signal to the associated circuitIF said comparison circuit indicates that an external power supply voltage has strengthened to a second voltage after previously having declined from a target voltage to a third voltage, but NOT IF said external power supply voltage has declined from said target voltage to a second voltage without having passed through said third voltage; and in said second mode, said reset control logic is permitted to output said reset signal to the associated circuit WHENEVER said comparison circuit indicates that said power supply voltage is less than said second voltage; wherein said target voltage is greater than said first voltage, said target voltage is greater than said second voltage, and said second voltage is greater than said third voltage; a power failure logic output connection, connected to be provide an externally accessible logic signal showing whether said comparison circuit indicates that said power supply voltage is less than said second voltage; at least one memory control signal input connection, connectable to receive memory control signals generated externally, and at least one corresponding memory control signal output connection, for connection to a control terminal of an SRAM memory, said memory control signal input being connected to said memory control signal output only if said power supply voltage is above said second voltage; a pushbutton input connection, for connection to a manually actuable external switch, said pushbutton input being connected so that said reset signal becomes active when the manually actuable external switch pulls said pushbutton input to ground; and a watchdog timer circuit, connected to monitor electrical activity on a predetermined connection, and, if no activity is detected on said external connection for at least a predetermined duration, to cause said reset signal to be output to the associated integrated circuit.
 20. The integrated circuit of claim 19, wherein said comparison circuit monitors said input voltage and said external power supply voltage from separate connections, and said input voltage does not necessarily correspond to said external power supply voltage; and wherein, in said second mode, said reset signal declines as said supply voltage declines; and wherein said reset signal includes both true and complemented signals. 